From FAQ:

Altered Pilot Skill Some abilities alter the pilot skill value of a ship. If several game effects alter the pilot skill of a ship, only the most recent effect is applied. For example, if a Green Squadron Pilot equips Veteran Instincts, it increases its pilot skill from 3 to 5. However, if it later receives the “Damaged Cockpit” Damage card faceup, its pilot skill is reduced from 5 to 0 (rather than 2). Next round, however, Wedge Antilles can use Swarm Tactics to raise its pilot skill to 9.

So if I equip Vader with VI, he'll go to PS11, then if I use Decoy on Echo, what PS does she get? 9 Echo and 6+2 Vader? Or 9+2 Echo and 6 Vader?

Thanks

Vader + VI = PS11.

Echo + Decoy = PS6.

Decoy allows you to swap pilot skill with a ship at range 1-2 at the start of combat. If you choose to do so, then ECHO is PS11 and Vader is PS6.

If something changes the PS later, like Damaged Cockpit on Echo, then you would be trading PS0 for PS11.